TEL AVIV — The U.S. and Israeli militaries began a combined air-defense exercise on Thursday involving about 1,000 American soldiers and simulating a scenario in which U.S. forces deploy to Israel to help defend the country against incoming missiles.

The three-week drill, the fifth since 2001, is part of a growing partnership between the two militaries that has coincided with rising fears in Israel about Iran’s growing arsenal of missiles and nuclear ambitions.

“In time of need the Israel Defense Forces will protect our country, however, if decided, our defenses will be enhanced by the United States’ capabilities,” Israeli Air Defense Corps commander Brig. Gen. Doron Gavish told a news conference.

Iran and Israel both confirmed Thursday that representatives of their governments attended a conference in Cairo last month focused on global nonproliferation issues, a rare joint appearance by officials from the enemy states.

According to two officials who attended the meeting, the Iranian and Israeli sides got involved in a “give and take” over the issue of Israel’s nuclear program. But the officials said the exchange wasn’t unlike other spats the two countries have had on the nuclear issue.

Israel has a formal position of neither confirming nor denying its possession of atomic weapons. “There was nothing particularly new here,” said one conference participant. “Israel refused to confirm or deny.”

Officials from both sides denied, however, that their governments engaged in any direct negotiations or one-on-one meetings.

“The Israeli member was there and the Iranian member was there, but it was not direct talks,” said Yael Doron, a spokeswoman for the Israeli Atomic Energy Commission. “They were in the same room around the table and that’s it.”

Israeli officials have voiced skepticism about recent reports that the U.S. and other powers are close to a deal with Iran to curb its nuclear program by exporting much of its nuclear fuel to Russia.

A senior Israeli defense official said this year’s drill was the largest yet, in part because the threat from Iran keeps growing. But U.S. and Israeli commanders leading the exercise have played down reports that the drill is meant to simulate a war with Iran or a potential nuclear attack.

“During the planning, the term nuclear was never brought into any of the discussions. It changes the way we fight,” said U.S. Army Col. Tony English, commander of the 357th Air Defense Brigade, which is based in Germany and is one of the lead units in the exercise.

The U.S. and Israeli militaries have strong historic ties. U.S. military aid to Israel in 2009 will total $2.55 billion. Only Iraq receives more. In 1991, during the Gulf War, the U.S. dispatched Patriot missile batteries to Israel in a largely failed attempt to intercept Iraqi Scud missiles targeted at Israel.

In July, when the U.S. and Israel aborted a test of a missile-defense shield under development by the two countries due to what they called a “glitch,” Israeli military analysts called the setback a psychological blow to Israel.

Last fall, a detachment of fewer than 200 U.S. soldiers from Col. English’s brigade deployed to southern Israel to run an advanced radar system expected to increase the range at which Israel can detect and ultimately intercept Iranian missiles. It was the first permanent deployment of U.S. forces to Israel.

The combined exercise began Thursday with U.S. forces deploying throughout Israel, and will be followed in coming days by a computer simulation in which Israeli and U.S. commanders work together to thwart an attack. The exercise will conclude with a live fire drill in which U.S. and Israeli forces attempt to shoot down 10 incoming warheads.

The U.S. has brought its full arsenal of missile-defense systems to Israel for the exercise, including 24 Patriot missile launchers and a Navy destroyer armed with the advanced Aegis Ballistic Missile Defense System.

The drill will also be a closely watched early test of the U.S.’s newest missile-defense system, known as THAAD, which was rolled out in Texas in May 2008 but isn’t yet fully operational.

The three U.S. missile-defense systems, working alongside Israel’s own missile defenses, are designed to form a multitiered shield capable of intercepting incoming missiles at various points along their trajectory, U.S. and Israeli military officials sai